grazing intensity effects on soil nitrogen mineralization in semi-arid grassland on the loess...
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ORIGINAL PAPER
Grazing intensity effects on soil nitrogen mineralizationin semi-arid grassland on the Loess Plateau of northernChina
Tianzeng Liu • Zhibiao Nan • Fujiang Hou
Received: 3 March 2011 / Accepted: 8 July 2011 / Published online: 5 August 2011
� Springer Science+Business Media B.V. 2011
Abstract Soil nitrogen transformation has been the
subject of growing attention in many semi-arid
grassland ecosystems. In our study, we employed an
intact soil core in situ incubation technique and
measured seasonal changes in soil net nitrogen
mineralization and nitrification rates. The measure-
ments were taken from the upper 0–10 cm soil layer
of a permanent grassland during a growing season in
a 8.5-year field experiment on the Loess Plateau,
China that had four grazing intensities (0, 2.7, 5.3 and
8.7 sheep ha-1). Our results demonstrate marked
seasonal variations in inorganic nitrogen pools, net
nitrogen mineralization and net nitrification. The
rates of mineralization and nitrification were highest
in August and lowest in September. No consistent
differences in monthly net nitrogen mineralization
and monthly nitrification rates were observed among
the different grazing intensities. Sheep grazing stim-
ulated nitrogen transformation, and the most stimu-
lation occurred at a heavy grazing intensity of 8.7
sheep ha-1. The mean soil net nitrification rate was
positively correlated with the soil C/N ratio and pH.
The mean N mineralization rate was negatively
correlated with soil organic carbon, but was posi-
tively correlated with the soil C/N ratio. Our study
demonstrated net nitrogen mineralization and nitrifi-
cation rates were strongly linked to grazing intensity,
soil temperature and moisture content.
Keywords Grazing intensity � Grassland � Inorganic
nitrogen � Mineralization � Nitrification
Introduction
Nitrogen availability and transformation processes
are key parameters for assessing grassland produc-
tivity. These parameters affect net primary produc-
tivity and species richness (Bai et al. 2004; Barger
et al. 2004). Nitrogen availability largely depends on
nitrogen mineralization and nitrification, which is the
biological process whereby organic nitrogen is con-
verted to inorganic forms (Liu et al. 2010). Nitrogen
mineralization and nitrification occur concurrently in
soils, and their balance often determines nitrogen
available for plant uptake or nitrogen lost via
denitrification and leaching. Because of the important
role of nitrogen mineralization and nitrification in
determining grassland productivity, a better under-
standing of how grazing intensities influence these
processes in soils will facilitate our predictions of soil
nitrogen dynamics and net primary production in
grassland ecosystems.
T. Liu � Z. Nan (&) � F. Hou
The State Key Laboratory of Grassland Farming Systems,
College of Pastoral Agriculture Science and Technology,
Lanzhou University, Lanzhou 730020, Gansu, China
e-mail: [email protected]
T. Liu
e-mail: [email protected]
123
Nutr Cycl Agroecosyst (2011) 91:67–75
DOI 10.1007/s10705-011-9445-1
Herbivores are an integral component of grasslands
and their effect at the ecosystem level may include
nutrient transformation and availability, due to feed-
backs between plant responses to grazing and nitrogen
cycling (Holland et al. 1992). Nitrogen mineralization
is often stimulated by grazing primarily through the
decomposition of nitrogen-rich livestock feces
(McNaughton et al. 1988; Singh et al. 1991; Bauer
et al. 1987). Furthermore, livestock grazing often
decreased litter inputs, whereas selective foraging
may cause changes in litter quality as a result of
changes in plant community composition (van Wijnen
et al. 1999; Olofsson et al. 2001). These changes can
indirectly influence nitrogen mineralization by affect-
ing plant litter decomposition rates and soil microbial
activities (Tracy and Frank 1998). Although the
effects of grazing on soil nitrogen mineralization
have been assessed in various ecosystems (Schmidt
et al. 1999; Cookson et al. 2002; Suldava and Huhta
2003), there are still significant gaps in understanding
the effects of grazing on soil nitrogen cycling
processes. Some studies indicated that grazing stim-
ulated net mineralization (Xu et al. 2007; Le Roux
et al. 2003), while other studies showed that grazing
reduced nitrogen mineralization (Accoe et al. 2004;
Biondini et al. 1998). One possible explanation for the
divergent results in the literature is that the grazing
intensities have varied between previous studies. In
this study the impact of different grazing intensities on
nitrification and mineralization is assessed.
The grassland in northwest in China represents the
typical arid and semiarid regional vegetation, and is
sensitive to climatic change and grazing disturbance
(Christensen et al. 2004). Grassland accounts for
57.4% of the total agricultural land areas on the Loess
Plateau. The grassland mainly includes steppe, aban-
doned cropland, shrubland, and some woodland. About
97% of the grasslands have degraded to various extents
and more than 1/3 of the total grassland area in the
region has severely degraded (Zhou et al. 2010).
Precipitation is the largest limiting factors for the
agricultural production. Sheep grazing is the main type
of land-use in the regional farming systems. Under-
standing the effects of grazing on nitrogen cycling in
these grassland ecosystems is critical for better man-
agement. The objectives of the present study were: (1)
to investigate the seasonal variation of inorganic
nitrogen pool and net nitrogen mineralization rate in
situ in surface soils and (2) to compare the difference in
soil nitrogen mineralization rate among the different
grazing intensities over a growing season.
Materials and methods
Site description
The experiment was conducted at Tianshui Grassland
Research Station of the College of Pastoral Agricul-
ture Science and Technology, Lanzhou University,
located in Huanxian County, Gansu Province,
Western China. The latitude of the experimental site
is 37.14N, the longitude is 106.84E, and the elevation
is 1,650 m above sea level. This area has a typical
semi-arid monsoon climate. The mean annual pre-
cipitation is 359 mm, more than 80% of which falls
during late June to mid-September. This period
broadly corresponds to the main crop and pasture
growing season. The mean annual temperature is
7.1�C. The soil at the study site is classified as
cambisol based on the FAO soil classification. The
grassland is principally composed of Stipa bungeana,
shrubby lespedeza (Lespedeza bicolor), wormwood
(Artemisia capillaris), flaccidgrass (Pennisetum flac-
cidum) and green bristle grass (Setaria viridis). The
growing season runs from May to October.
The study site was overgrazed for many years, and
in 2001 the area was fenced off to establish an
experimental trial. In autumn 2001, 12 experimental
plots (100 m 9 50 m) were selected and fenced in
native steppe grassland with visually similar vegeta-
tion botanical composition and cover and similar
slope and aspect. Four grazing treatments were
maintained for 8.5 years from 2001 to 2010, the year
of this study. There were 0, 4, 8 and 13 Tan sheep
lambs grazing rotationally in three replicated 0.5 ha
plots, representing the stocking rates of 0, 2.7, 5.3 and
8.7 sheep ha-1, respectively. Tan sheep lambs were
purchased each spring at approximately 20 kg body
weight. Every year, grazing started in early June and
ended in early September. Each plot was rotationally
grazed three times per year, each time for 10 days
with a rotation interval 30 days (Table 1). All sheep
used in the current study had similar body condition
at the start of each year’s experimental period. During
the experiment, sheep grazed the plots each day and
were returned to a shed each night. Soil character-
istics, in spring 2010, in the top 10 cm soil layer
68 Nutr Cycl Agroecosyst (2011) 91:67–75
123
under different grazing intensities are presented in
Table 2.
Field soil incubation and sampling
From 1st June to 1st October 2010, we took monthly
measurements of soil net nitrogen mineralization
under four grazing intensities, using an intact core in
situ incubation technique. At the start of incubation
period, aboveground vegetation was clipped at the
ground level and removed together with litter. Soil
cores (positioned in pairs) were taken using polyvinyl
chloride PVC tubes (12 cm height and 7.5 cm in
diameter) from 6 random locations in each plot. One
of each pair tubes (initial sample) was inserted 10 cm
into the ground, then it was removed and returned to
the laboratory in an icebox to determine initial soil
ammonium (NH4?–N) and nitrate (NO3
-–N) con-
centrations. The second tube of each pair (incubated
sample) was inserted 12 cm into the ground and
drawn out immediately and a soil layer about 2 cm
thickness was scraped out from the bottom. This free
space was filled with a mesh nylon bag containing
anion exchange resin for the continuous collection of
nitrate leached. The nylon bag was fixed with a
0.4 cm thick polyurethane foam disk that was finally
pressed into the tube before its reintroduction into the
soil hole for exposure in the field. At the end of the
each incubation period, the incubated samples were
extracted and the nylon bags collected separately.
Soil samples and the resin bags were stored in a
refrigerator at 4�C until analyzed.
Soil samples and resin bags analysis
In the laboratory, following removal of roots and
stones, each soil core was well mixed by hand to form
a homogenous sample and then passed through a
2 mm sieve. To analyze the inorganic nitrogen, a
10 g aliquot subsample was taken from each of the
initial and incubated soil cores, and then 50 ml of
2 M KCl solution was added. The soil and extractant
were shaken for 1 h in a reciprocal shaker. After
shaking, the soil suspension was filtered through
Whatman No. 1 filter paper. The resin bags were
washed with deionized water and dried at room
temperature prior to extraction. The resin bags were
extracted with 1 M NaCl solution, and then they were
washed with fresh NaCl solution (Liu et al. 2010).
The filtrates of soil and the solution of extracted resin
bags were kept frozen before they were analyzed for
NH4?–N and NO3
-–N on a FIAstar 5000 Analyzer.
The net mineralization and nitrification rates were
expressed on a dry mass basis.
Measurement of environment factors
Soil moisture content of the sample from each plot
was determined by drying the fresh soil at 105�C for
24 h. The air-dried soil samples were used for
measuring pH, organic carbon content and total
Table 1 Experimental design for different grazing intensities
Treatment Livestock
numbers
(head)
Plot
area
(ha)
Grazing
period
(days/month)
Stocking
rate
(sheep/ha)
Control 0 0.5 10 0
Light grazing 4 0.5 10 2.7
Moderate
grazing
8 0.5 10 5.3
Heavy
grazing
13 0.5 10 8.7
Table 2 Dominant species, aboveground biomass and soil characteristics under different grazing intensities (GI)
GI (sheep ha-1) 0 2.7 5.3 8.7
Dominant species
aboveground biomass (g m-2)
Stipa bungeana 5.11 ± 1.29 8.58 ± 1.74 13.55 ± 1.69 12.35 ± 1.78
Lespedeza bicolor 20.12 ± 1.45 13.83 ± 1.11 9.97 ± 0.63 3.40 ± 0.98
Artemisia capillaris 61.09 ± 1.52 39.08 ± 2.89 17.62 ± 1.20 17.17 ± 1.31
Total aboveground biomass (g m-2) 133.82 ± 13.56 117.25 ± 9.44 104.52 ± 9.65 71.28 ± 3.84
Total N (g kg-1) 0.22 ± 0.01 0.21 ± 0.04 0.18 ± 0.03 0.14 ± 0.03
Soil organic C (g kg-1) 5.87 ± 0.23 5.77 ± 0.17 5.76 ± 0.13 5.71 ± 0.43
pH 8.41 ± 0.02 8.42 ± 0.01 8.41 ± 0.05 8.46 ± 0.01
C/N ratio 26.58 ± 1.80 27.41 ± 1.36 32.01 ± 1.94 40.62 ± 2.71
Nutr Cycl Agroecosyst (2011) 91:67–75 69
123
nitrogen content. Soil pH values were determined in a
water suspension (water:soil = 2.5:1). Soil organic
carbon content was analyzed using the H2SO4–
K2CrO7 oxidation method (Nelson and Sommers
1982). Soil total nitrogen (TN) contents were deter-
mined using the Kjeldahl acid-digestion method with
an autoanalyzer (Foss Inc., FIAstar5000, Sweden).
Daily rainfall and soil temperature (at a 10 cm
depth) were recorded at a meteorological station,
which is 1 km from the study site. Soil temperature at
10 cm depth in each replicate plot was measured
during a 30-day period in a pre-experiment. There
was no significant difference in daily mean soil
temperature between treatment plots. We therefore
did not measure the soil temperature at the experi-
ment site during the experiment. Instead, the soil
temperature from the meteorological station was
used.
Herbage mass and botanical composition data
were collected to provide a contextual background
for the grazing treatments. The aboveground herbage
mass (comprising green biomass, standing dead
material and litter on the soil surface) was determined
by clipping plants at the ground level from 1 m2
quadrats with four quadrats per plot. Samples were
oven-dried at 70�C for 48 h.
Calculations and data analysis
Net soil mineralization and nitrification rates per
incubation interval were defined as the amount of
mineral nitrogen (NH4?–N ? NO3
-–N) in the incu-
bated sample minus the amount of mineral nitrogen
in the initial sample. The net mineralization rate
(RM), and net nitrification rate (RN) (mg NO3-–
N kg-1d-1) was, given by:
RM ¼ Tm1 � Tm0ð Þ=t½ �;
and
RN ¼ Tn1 � Tn0 + Tresinð Þ=t½ �;
where Tm1 and Tn1 (mg kg-1) represent the total
inorganic nitrogen (NH4?–N ? NO3
-–N) and
NO3-–N concentrations after incubation, respec-
tively. Tm0 and Tn0 represent the initial total
inorganic nitrogen (NH4?–N ? NO3
-–N) and
NO3-–N concentrations before incubation, respec-
tively. Tresin represents the concentration of NO3-–N
in the resin, t is the number of incubation days.
The data were checked for assumptions of nor-
mality and validate ANOVA approach. Repeated
measures ANOVA in a generalized linear model was
used to examine the differences of inorganic nitrogen
concentrations and net nitrogen transformation rates
under different grazing intensities, using grazing
intensity and sampling date as main effects. One-
way ANOVA was used for the comparison of the
differences in mineralization rates from grazing
intensity for each incubation period. LSD multiple
test was applied to separate treatment means. Pearson
correlation analysis was applied to test the relation-
ships of nitrogen mineralization and nitrification rates
with climatic factors and soil characteristics, respec-
tively. All statistical procedures were performed in
SPSS 13.0 (SPSS Inc., Chicago, IL, USA). All results
are reported as mean ± standard error on a dry soil
basis.
Results
Environmental conditions, plant biomass and soil
properties
Over the period of field incubation, the monthly soil
temperature at 10 cm depth ranged from 11.2�C in
October to 26.7�C in July (Fig. 1). This was a much
drier year, with precipitation less than 45.1% of the
mean annual precipitation. Soil moisture content
under various grazing intensities showed similar
patterns. There were significant differences in soil
moisture content between the four grazed treatments
(Fig. 2). Mean soil moisture content in the ungrazed
treatment was 25% higher than that under the grazing
intensity of 8.7 sheep ha-1. Total nitrogen, organic
carbon, pH and C:N of the top 10 cm soil layer were
listed in Table 2.
Temporal patterns of soil mineral nitrogen pools
Soil NH4?–N concentrations ranged from 7.11 to
31.74 mg kg-1 and peaked in August for all grazing
intensities (Fig. 3a). The heavy grazing treatment had
a significantly higher concentration of NH4?–N than
other three grazing treatments (Table 3). Soil NO3-–
N concentration ranged from 1.16 to 8.29 mg kg-1
and showed a gradually increasing trend throughout
the period of field incubation. The highest NO3-–N
70 Nutr Cycl Agroecosyst (2011) 91:67–75
123
concentration occurred in August and lower in
September in all the grazing treatments (Fig. 3b).
The inorganic nitrogen concentrations in surface soil
layer ranged from 9.78 to 40.84 mg kg-1. Soil
inorganic nitrogen, NH4?–N and NO3
-–N under the
four grazing intensities showed similar temporal
patterns (Fig. 3c). NH4?–N was the dominant inor-
ganic nitrogen. The concentrations of soil NH4?–N
and NO3-–N were strongly affected by sampling
times and grazing intensities (Table 3).
Soil net nitrogen mineralization and nitrification
rates
The soil net nitrogen mineralization and nitrification
rates under the four different grazing intensities
exhibited significant temporal variations during the
growing season. Net nitrogen mineralization rates
ranged from -0.14 to 1.31 mg kg-1 d-1, and were
higher in August, lower in June, July, and September,
respectively (Fig. 4a). During the incubation period,
the soil net nitrogen mineralization rates under the four
grazing intensities had increased till August, then
declined in September. (Fig. 4a). There was net
nitrogen immobilization during September. Consistent
higher values in net nitrogen mineralization rates were
observed under the grazing intensity of 8.7 sheep ha-1
(Fig. 4b). The soil net nitrogen nitrification rates were
generally low and varied from -0.14 to 0.29 mg kg-1
0
10
20
30
40
50
60
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Prec
ipita
tion
(mm
)
-10
-5
0
5
10
15
20
25
30
Soil
tem
pera
ture
(
)
Fig. 1 Changes in mean
monthly soil temperatures
(0–10 cm depth, points) and
precipitation (bars) of the
study site during 2010
0
1
2
3
4
5
6
7
Jun Jul Aug Sep
Soil
moi
stur
e(%
)
02.75.38.7
Fig. 2 Changes in soil moisture content in the top 10 cm soil
layer under different grazing intensities
0
2
4
6
8
10
NO
3- -N (
mg
kg-1
) (b)
0
10
20
30
40
50
NH
4+ -N +
NO
3- -N
(c)
0
5
10
15
20
25
30
35
40
Jun Jul Aug Sep
NH
4+ -N(m
g kg
-1)
02.75.38.7
(a)
Fig. 3 Seasonal patterns of NH4?–N a, NO3
-–N b and total
mineral nitrogen c in the top 10 cm soil layer under different
grazing intensities. Each point is the mean from three
replicated plots. Error bars represent ±SE. 0, 2.7, 5.3 and
8.7 sheep ha-1 denote different stocking rates. Each plot was
rotationally grazed three times per year, each time for 10 days
with a rotation interval of 30 days
Nutr Cycl Agroecosyst (2011) 91:67–75 71
123
d-1. Nitrate nitrogen immobilization occurred during
September, especially in the heavy grazing treatment.
Long term grazing significantly increased net nitrifi-
cation compared to the ungrazed plots (P \ 0.05).
Repeated measures indicated that grazing intensity
significantly affected soil mineral nitrogen dynamics.
Inorganic nitrogen concentrations and net nitrogen
transformation rates were also significantly affected
by grazing intensity, sampling date and the interac-
tion of grazing intensity and sampling date (Table 4).
Grazing effects on net nitrogen mineralization rates
were significant during July and September. Net
nitrification rates were significantly affected by
grazing treatments during August and September.
Relationships between nitrogen mineralization
and nitrification and soil characteristics
Soil net nitrogen mineralization and nitrification rates
showed complex relationships with soil characteris-
tics. The mean soil net nitrification rate under the four
grazing intensities was positively correlated with the
soil C/N ratio and pH, respectively (Table 5). The
mean nitrogen mineralization rate was negatively
correlated with soil organic carbon, but was posi-
tively correlated with the soil C/N ratio (Table 5).
Discussion
Temporal patterns of soil nitrogen mineralization
rate
Our study showed substantial temporal variations in
the soil net mineralization rates under the four grazing
intensities during a growing season, a peak of nitrogen
mineralization in August, and a substantial amount of
nitrogen immobilization in September when soil
moisture content and temperature were low (Gleeson
et al. 2008). Nitrogen mineralization is microbe-
governed processes. The net nitrogen mineralization
increased with higher temperatures and raised soil
moisture, as these two characteristics facilitate greater
microbial activities. The increase in root mortality and
aboveground litter from senescence may have con-
tributed to the nitrogen immobilization in September,
because the root mortality provides more available
Table 3 Comparisons of the mean soil moisture, mineral nitrogen pool (NH4?–N ? NO3
-–N), nitrogen mineralization rate and
nitrogen nitrification rate among different grazing intensities plots
Variables Grazing intensity (Sheep ha-1)
0 2.7 5.3 8.7
Soil moisture (%) 5.00 ± 0.67a 4.29 ± 0.62b 4.03 ± 0.58b 3.62 ± 0.61c
NH4?–N (mg kg-1) 16.49 ± 3.95b 18.46 ± 4.88b 17.83 ± 3.92b 21.66 ± 4.39a
NO3-–N (mg kg-1) 3.84 ± 1.25b 3.76 ± 1.47b 3.95 ± 1.09b 4.45 ± 1.39a
Net mineralization rate (mg kg-1 d-1) 0.07 ± 0.10b -0.08 ± 0.07c -0.04 ± 0.05c 0.11 ± 0.10a
Net nitrification rate (mg kg-1 d-1) 0.14 ± 0.06b 0.13 ± 0.05b 0.15 ± 0.04b 0.22 ± 0.06a
Significant differences among the grazing intensities plots are indicated by different letters at P \ 0.05
Fig. 4 Changes in soil net nitrogen mineralization rate and net
nitrification rate in the top 10 cm soil layer under different
grazing intensities. Each column is the mean from three
replicated plots. Error bars represent ±SE. Treatments with
different letters are statistically different at P \ 0.05 level. 0,
2.7, 5.3 and 8.7 sheep ha-1 denote different stocking rates.
Each plot was rotationally grazed three times per year, each
time for 10 days with a rotation interval of 30 days
72 Nutr Cycl Agroecosyst (2011) 91:67–75
123
carbon to stimulate microbial activities, which
resulted in more mineral nitrogen to be immobilized
by soil microbes (Luizao et al. 1992).
Soil NH4?–N and NO3
-–N concentrations of all
grazing treatments increased throughout the entire
growing season except for the last sampling in early
October when soil mineral nitrogen declined due to a
marked increase in immobilization. These patterns
were consistent with the dynamics of soil moisture
content, suggesting that dynamics of soil moisture
content played a key role in controlling soil nitrogen
nitrification and mineralization. One possible expla-
nation for this is that the experimental region is located
in a semiarid region where the soil moisture content of
topsoil was usually low for a long period of time. Our
results indicate that grazing intensities had less
influence on the monthly dynamics of soil nitrogen
transformation than temperature and moisture condi-
tions. Soil temperature and moisture content have been
consistently reported to control soil nitrogen mineral-
ization (Dalias et al. 2002). The marked differences in
soil moisture contents can directly affect soil nitrogen
mineralization by modifying soil water availability
which controls microbial activity (Orchard and Cook
1983). Other previous studies also showed the highest
nitrogen mineralization rate usually occurred in sum-
mer and coincided with higher temperature and soil
moisture conditions, and then decline in fall (Zhang
et al. 2008). Some studies indicated that oxygen supply
and temperature usually play more significant roles in
nitrogen transformations in wet ecosystems (Paul et al.
2003). Whereas soil nitrogen transformations was
sensitive to soil moisture content in arid and semi-arid
ecosystems (Xu et al. 2007; Vangestel et al. 1993).
Effects of grazing on soil nitrogen mineralization
rate
The relationship between grazing intensities and net
nitrogen mineralization rate is challenging to predict
within a whole system context that includes abiotic
driving variables and many biotic factors (Seagle and
McNaughton 1993; Leriche et al. 2001). The heavy
grazing treatment in this study increased the soil
nitrogen mineralization rate when compared with the
response observed in the ungrazed and light grazing
treatments. However, this pattern was not apparent
for the monthly net nitrogen mineralization rate.
Effects of soil temperature and moisture content on
the monthly nitrogen dynamics were stronger than
grazing intensities. A survey of multi-year studies is
needed to further test this pattern, considering the low
precipitation (only 45.1% of the mean annual
precipitation) during the year of this study.
Different plant species that exhibit contrasting
growth rates and root functioning can have different
influences on the microbially mediated processes such
as mineralization and nitrification (Zhao et al. 2010).
However, grazing strongly modifies the identity of
major plant species (Collins et al. 1998). In our present
study, marked changes in plant species composition
were observed in response to the grazing intensities.
Table 4 Results of repeated measures ANOVA of grazing intensities and sampling date on soil moisture, NH4?–N, NO3
-–N, net
nitrogen mineralization and nitrification rates in grassland
Source Soil moisture Soil mineral N pool Net rate
NH4?–N NO3
-–N Mineralization Nitrification
Grazing intensities 0.014 0.025 0.033 0.000 0.003
Sampling date 0.000 \0.001 \0.001 \0.001 \0.001
Grazing intensities * date 0.021 0.000 0.037 0.002 0.027
Table 5 Pearson correlation coefficients (r) of mean soil net nitrogen mineralization and nitrification rates with soil characteristics in
a semiarid grassland
r Total N Soil organic C C/N ratio pH
Net mineralization rate 0.234 -0.569* 0.612* 0.293
Net nitrification rate -0.426 -0.190 0.567* 0.659*
* P \ 0.05
Nutr Cycl Agroecosyst (2011) 91:67–75 73
123
Significantly lower aboveground biomass of the three
dominant species was found under the heavy grazing
intensity (8.7 sheep ha-1) than under other grazing
intensities. The increase in mineralization was attrib-
uted to mineral nitrogen accumulated. Because there
was not more plant uptake in the heavy grazing plots,
resulting in higher level of mineral nitrogen concen-
tration. Grazing stimulation of soil net nitrogen
mineralization rate was also attributed to enhanced
rhizosphere microbial activities (McNaughton et al.
1997). This mechanism was considered that grazing
increased labile carbon availability by a stimulation of
root exudates that promote rhizosphere microbial
metabolism and result in an enhanced nitrogen min-
eralization (Paterson and Sim 1999; Hamilton and
Frank 2001).
Grazing has been reported to promote soil miner-
alization in some grassland ecosystems, partly as a
result of dung and urine deposition (Barger et al.
2004; Xu et al. 2007; Tracy and Frank 1998). The
positive direct effect of herbivore on nitrogen cycling
is based upon assumptions of reductions in below-
ground carbon input, increases in the quality of soil
organic matter and deposition of high-nitrogen prod-
ucts (Andrioli et al. 2010; Knops et al. 2002).
Livestock only use a small proportion of the nutrients
they ingest, and 60–95% of ingested nutrients are
returned to the pasture in the form of dung and urine
(Haynes and Williams 1999). In the current study,
high concentration of nitrogen was found under the
heavy grazing intensity, largely due to organic matter
and nutrient transfer via animal excreta. In the long
term, this uneven distribution of nutrients due to
sheep grazing adds might contribute to the mineral-
ization. Soil microbial biomass and activity can be
stimulated by available C supply due to the incorpo-
ration of animal excreta into soil (Iyyemperumal
et al. 2007). An increase in soil microbial activity is
also likely to boost the mineralization and turnover of
soil organic matter.
Conclusions
Grazing intensity over the long-term is an important
factor in regulating nitrogen transformation in the
semi-arid grassland of Loess Plateau. There are
significant differences in net nitrogen mineralization
rates between different grazing intensity treatments.
However, grazing intensity play a less role in
regulating monthly nitrogen transformation rates than
soil moisture content and temperature. The long-term
grazing under highest intensity (8.7 sheep ha-1)
substantially reduce aboveground plant biomass and
increase mineral nitrogen, which indicates that the
system under the highest grazing intensity can not be
sustained. If the objective for the Loess Plateau is to
find the optimal balance between the production and
ecology of grasslands, then control of grazing inten-
sities is required to accommodate that goal, because
heavy grazing can reduces pasture productivity and
soil nitrogen storage.
Acknowledgments We thank the staff of Tianshui Grassland
Research station for providing the temperature and
precipitation data. We appreciate the critical and constructive
comments from two anonymous reviewers. This research was
financially supported by the National Basic Research Program
of China (2007CB108902).
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